1. Field of the Invention
The present invention relates to photosensitive members for electrophotography and, more particularly, to photosensitive members for electrophotography comprising a photosensitive layer formed on a conductive substrate, the photosensitive layer comprising a powdered photoconductive material dispersed in a binder of an insulating high molecular material.
2. Description of the Prior Art
In electrophotography, a photosensitive member is charged and then exposed to a light pattern to form a latent electrostatic image on a surface of its photosensitive layer, and the latent image is developed with a powder developer into a visible powder image, transferred to a sheet of suitable transfer paper such as plain paper, and then fixed thereon by fusing to form a copy. The photosensitive member is then cleaned and re-used for copying other subject matter. In a modified electrophotographic process, the latent image formed on the surface of the photosensitive layer is directly fixed on the photosensitive member by fusing to form a final print.
The photosensitive members for electrophotography are generally required to have various electrophotographic and physical properties. The important electrophotographic properties include photosensitivity, dark decay, residual potential, and fatigue in repeated use, and the physical properties include abrasion resistance and humidity resistance.
In commercial electrophotographic copying apparatuses, there have been employed photosensitive members utilizing amorphous selenium, cadmium sulfide or zinc oxide as a photoconductive material. The photosensitive members utilizing amorphous selenium are generally produced by depositing selenium on a surface of a conductive substrate with a plate or drum form. It is, however, difficult with selenium to produce photosensitive members with high quality because the properties of a selenium photosensitive layer may vary over a wide range, depending on the conditions of deposition. This causes a considerable increase in the manufacturing cost. Also, amorphous selenium photosensitive layers are poor in flexibility and require care in handling because of toxic properties of selenium.
The other photosensitive members utilizing zinc oxide or cadmium sulfide may be produced by coating on a surface of a substrate with a dispersion of a photoconductive material in an insulating resin binder. Such photosensitive members, generally called a binder-type, may vary in their electrophotographic properties, depending on an amount of the resin binder used. For example, the photosensitivity decreases with an increase in the amount of the used binder. It is therefore required to decrease the amount of the binder as much as possible. However, this causes a deterioration of physical properties including flexibility, surface smoothness, hardness and abrasion resistance of the photosensitive layers. Also, the binder-type photosensitive members become worse in electrophotographic properties due to the influence of ozone generated by corona discharging, and have a serious problem in sanitation in that there is a fear of environmental pollution because of toxic properties of cadmium and zinc oxide.
To overcome these disadvantages, there have been proposed various photosensitive members utilizing non-toxic organic photoconductive materials. For example, photosensitive members for electrophotography utilizing phthalocyanine and its compounds have been proposed and described in specifications of pending patent applications, for example, Japanese patent laid-open Nos. Sho 50-38543, 51-95852, 53-64040, 53-83744 and 54-147838. It is known that these photosensitive members have superior processing characteristics, high photosensitivity and possess no environmental pollution, and that they are highly sensitive even to light with a long wavelength, such as laser lights emitted from semiconductor lasers. These binder-type photosensitive members are greatly affected in the electrophotographic properties by the kinds of resin binder to be combined with phthalocyanines. It is therefore required to choose such a resin binder that does not cause deterioration of electrophotographic properties such as charging properties, photosensitivity, dark decay characteristics and reusability.
The resins conventionally used for the binder-type electrophotographic members are silicon resins, epoxy resins, alkyd resins, polyester resins, polycarbonate resins, vinyl resins, thermoplastic acrylic resins and urethane resins. However, such resins have been proved to be inadequate as a binder for production of photosensitive members utilizing phthalocyanine or its derivative. When the photosensitive layers are produced from phthalocyanines dispersed in the conventionally used resin, electrons generated in the layer are slow in being transported to the surface to the photosensitive layer for dissipation of surface charges, thereby requiring a relatively long time to form a latent image. This phenomenon is known as an induction effect and must be prevented from occurring when such a photosensitive layer is to be used for a high speed copying apparatus.
Also, when the conventionally used resins are employed as a binder for photosensitive members utilizing phthalocyanines, electrophotographic properties of the member deteriorate remarkably with increase of cycles of the repeated use. For example, the use of silicon resins provides low charging characteristics and the use of epoxy resins deteriorates photosensitivity. With urethane resins the members are low in the photosensitivity, but high in fatigue in repeated use, though they are high in the charge-discharge characteristics, dark decay characteristics and adhesive strength.
It has also been proposed to use thermoplastic acrylic resins as a binder for photosensitive members utilizing phthalocyanines. The combined use of thermoplastic acrylic resins and phthalocyanines enables the production of photosensitive members with better electrophotographic characteristics as compared with those utilizing conventional resins. However, it is difficult even with thermoplastic acrylic resins to prevent the member from the filming phenomenon that a toner film is formed on the surface of the member during the repeated use. This causes deterioration of the image qualities of prints. Also, it is not possible to obtain the electrophotographic properties and abrasion resistance sufficient for practical uses.